Modified Fisher Scale Calculator (SAH Vasospasm Risk on CT)
Grade the first CT after subarachnoid hemorrhage with the modified Fisher scale (0–4) from the thickness of subarachnoid blood and intraventricular hemorrhage, with the risk of symptomatic vasospasm.
How it works
Grade 0: no SAH or IVH · Grade 1: focal or diffuse thin SAH, no IVH · Grade 2: thin SAH with IVH · Grade 3: thick SAH (completely filling at least one cistern or fissure), no IVH · Grade 4: thick SAH with IVH. IVH = blood in both lateral ventricles. Symptomatic vasospasm (Frontera 2006): 1 ≈ 24%, 2 ≈ 33%, 3 ≈ 33%, 4 ≈ 40%.
The original Fisher scale (1980) graded the first CT after aneurysmal subarachnoid hemorrhage to predict vasospasm, but its highest grade grouped intraventricular and intracerebral blood with thin SAH, so risk did not rise steadily. Frontera and colleagues (Neurosurgery 2006) derived the modified Fisher scale in 1,355 patients: thick cisternal blood and blood in both lateral ventricles each independently raised the risk of symptomatic vasospasm, giving a stepwise scale from 0 to 4. It guides how closely patients are monitored for delayed cerebral ischemia, usually with transcranial Doppler and neurological checks between days 3 and 14.
Worked example
A CT shows a thin diffuse layer of subarachnoid blood and blood layering in both lateral ventricles: modified Fisher Grade 2 — thin SAH with IVH, with about 33% symptomatic vasospasm in the derivation cohort.
Sources: Frontera JA et al. Prediction of symptomatic vasospasm after subarachnoid hemorrhage: the modified Fisher scale. Neurosurgery 2006;59:21–27 · Fisher CM, Kistler JP, Davis JM. Relation of cerebral vasospasm to subarachnoid hemorrhage visualized by CT scanning. Neurosurgery 1980;6:1–9
Frequently asked questions
What is the difference between the Fisher and modified Fisher scales?
The modified scale counts thick cisternal blood and intraventricular blood as separate risk factors, so the risk of vasospasm rises with each grade. In the original Fisher scale, grade 4 (intracerebral or intraventricular clot) actually had a lower vasospasm rate than grade 3.
What counts as thick subarachnoid blood?
Blood that completely fills at least one cistern or fissure on the first CT. A thinner layer, focal or diffuse, is graded as thin SAH.
When does vasospasm happen after SAH?
Usually between days 3 and 14 after the bleed, peaking around days 7–10, which is why patients are monitored in a neurocritical care unit during this window.
Does a low grade mean no vasospasm?
No. Even grade 1 had about a 24% rate of symptomatic vasospasm in the derivation cohort; the grade sets the level of vigilance, not a guarantee.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside clinical judgement — not a diagnosis and not a substitute for medical assessment. Scores and cut-offs come from the cited validation studies and may not apply to every patient; follow local protocols.